I think at this point most direct changes to DRK's single target mitigation are going to result in a net nerf in terms of recasts. Oblation doesn't really have a proper niche as a tank mitigation tool, though, especially when you consider that several melee dps get personal 10% mitigation on half the recast from actions that they obtain 30 levels earlier. If 'Oblation' is an offering, perhaps the effect needs to be changed to empower the next defensive/utility action to have either an alternative or more potent effect. That would give you the primary strength of Heavensward's Dark Arts system without any of its design flaws. It could even be a way of working around DRK's magic-only effects. As some examples:

Dark Mind + Oblation: 15% physical damage reduction, increases recast to 90 seconds.
Dark Missionary + Oblation: 10% physical damage reduction and increases movement speed, increases recast to 120 seconds.
TBN + Oblation: 30% HP flat damage reduction, removes MP cost, restores HP on shield break without granting Dark Arts, increases recast to 30 seconds.
Blood Weapon + Oblation: Restores HP in addition to MP and blood gauge.

Having it on set charges would force you to be selective on how you wanted to augment your cooldowns on a fight, especially since most of these are on 60+ second recasts to begin with.